American SSTO VTOVL orbital launch vehicle. General Dynamics Space Systems Division proposal for the 1990 SDIO competition was a VTOL SSTO named Millennium Express. The final vehicle was a 15 degree cone with a 20%-length Rocketdyne aerospike engine. Payload was specified as 4500 kg into a polar low earth orbit. The Express could carry on its nose a payload module, a small Apollo-type two-crew separable manned capsule, or a six-crew module that remained attached to the vehicle for recovery. The similar Douglas Delta Clipper was selected by the USAF for further development.

The proposed subscale 'x-plane' version (equivalent to the Douglas DC-X) was dubbed Pathfinder. GD estimated that the Pathfinder version could be flying by 1993, with the prototype of the orbital version working up to orbital speed in a series of flights in 1995-1998. Production vehicles would fly from 1999. Derivatives of the basic vehicle could be combined with upper stages to support lunar bases, provide landing craft for lunar or Mars expeditions. The vehicle was to be designed for 500 flights, and have a ground turnaround time of 104 hours, using only 50 people.

General Dynamics and its partner Rocketdyne went through a series of iterations to arrive at the final design. The Model 106 of December 1990 was powered by RL10 engines (four in the small Pathfinder X-vehicle). This was replaced at the time of final proposal by the SX-107 pressure-fed plug nozzle. It was felt that if a Phase II contract had been received, the actual solution to fly would be the SX-109 with a pump-fed aerospike. In the event the aerospike technology did not prove itself in the test or prototype vehicles, fallback designs using conventional RS-44 engines were drawn up.